Magnetic tape recording system having tape with defined remanent magnetization

ABSTRACT

A magnetic recording tape exhibits reduced magnetic flux modulation and improved signal-to-noise ratio. Providing a magnetic recording layer with a more uniform thickness can improve the magnetic flux modulation characteristic. For example, the magnetic recording tape may have a coercivity of greater than or equal to 2000 Oe with a magnetic flux modulation characteristic having a one sigma standard deviation of less than 0.06. In some cases, the magnetic flux modulation characteristic may have a one sigma standard deviation of less than 0.05, or even 0.04. Reduced magnetic flux modulation can support increased storage densities.

This application is a divisional application of U.S. application Ser.No. 10/238,161 entitled “MAGNETIC RECORDING MEDIUM” filed Sep. 10, 2002now U.S. Pat. No. 6,960,385, which is incorporated herein by referencein its entirety.

TECHNICAL FIELD

The invention relates to data storage media and, more particularly, tomagnetic recording media and manufacture of magnetic recording media.

BACKGROUND

Magnetic recording media are widely used in a variety of data storageapplications and may take the form of magnetic tape or magnetic disks. Amagnetic recording medium generally includes a magnetic recording layerthat is coated onto a non-magnetic substrate. One or more intermediatelayers, such as a carrier or “sub” layer, may reside between themagnetic recording layer and the substrate.

Larger data storage demands and technological advancements have led toincreased data storage densities. Data ordinarily is recorded along oneof several tracks formed on the magnetic recording layer. To achieveincreased data storage density, magnetic media are designed toaccommodate a higher number of tracks and a higher number ofmagnetizations along the length of each track.

With increased data storage densities, maintenance of sufficientsignal-to-noise ratios for effective readout of the more closely packedmagnetizations has become a challenge. In particular, acceptable readoutperformance has required adherence to more aggressive drive andrecording medium tolerances and improved process control.

SUMMARY

The invention is directed to a magnetic recording medium having reducedmagnetic flux modulation and improved signal-to-noise ratio. Themagnetic recording medium may take the form of a magnetic tape or amagnetic disk.

Magnetic flux is a function of the remanent magnetization (M_(r)) of arecording layer and the recording layer thickness. The remanentmagnetization is the permanent magnetization that remains in a magneticmaterial forming a magnetic recording layer after an external magneticfield is removed. Hence, the remanent magnetization supports recordingof data. Modulation of the recording layer thickness along the recordingpath can impact the signal-to-noise ratio of data recorded on and readfrom the magnetic recording medium by producing magnetic fluxmodulation.

Providing a magnetic recording layer with a more uniform thickness alongthe recording direction of the magnetic recording medium, in accordancewith the invention, can improve the magnetic flux modulationcharacteristic. The recording direction may refer to the direction oftracks on a tape or disk. A wet-on-wet coating technique such as slidecoating or slot coating may be especially useful in achieving desireduniformity of thickness in the magnetic recording layer. Slide coating,in particular, may offer enhanced motion quality and coating uniformity,as will be described.

For example, the invention may provide a magnetic recording mediumhaving a coercivity of greater than or equal to 2000 Oe with a magneticflux modulation characteristic having a one sigma standard deviation ofless than 0.06. In some embodiments, the magnetic flux modulationcharacteristic may have a one sigma standard deviation of less than0.05, or even 0.04. The reduced magnetic flux modulation can supportincreased storage densities and enhanced recording performance.

In one embodiment, the invention provides a magnetic recording mediumcomprising a substrate and a magnetic recording layer formed over thesubstrate. The magnetic recording layer has a magnetic flux modulationcharacteristic with a standard deviation of less than or equal toapproximately 0.06.

In another embodiment, the invention provides a magnetic recordingmedium comprising a substrate, a carrier layer formed over thesubstrate, and a magnetic recording layer formed over the carrier layer.The magnetic recording layer has an average thickness of less than orequal to 0.15 microns, a coercivity of greater than 2000 Oe and amagnetic flux modulation characteristic with a standard deviation ofless than or equal to approximately 0.06.

In an added embodiment, the invention provides a magnetic tape recordingsystem.

The system comprises a magnetic recording tape with a magnetic recordinglayer having a magnetic flux modulation characteristic with a standarddeviation of less than or equal to approximately 0.06, a magneticrecording head, and a controller that controls the magnetic recording toread data from and write data to the magnetic recording medium.

In a further embodiment, the invention provides a method comprisingflowing a first fluid coating formulation over a first slide coatingsurface of a slide coater, flowing a second fluid coating formulationcontaining metal magnetic recording particles over a second slidecoating surface of the slide coater and over the first coatingformulation, the first and second coating formulations forming amulti-layer coating, and flowing the multi-layer coating onto a movingsubstrate to simultaneously apply the first and second coatingformulations to the substrate. The method further comprises controllingthe second fluid coating formulation to have, upon drying, a magneticflux modulation characteristic with a standard deviation of less than orequal to approximately 0.06.

The invention may provide one or more advantages. For example, with areduced magnetic flux modulation characteristic, data can be read fromthe magnetic recording medium with an improved signal-to-noise ratio.Reduced magnetic flux modulation can be achieved by carefullycontrolling the thickness uniformity of a magnetic recording layer inthe magnetic recording medium along the direction of recording. Ingeneral, a magnetic recording layer with a more uniform thickness iscapable of providing a magnetic flux modulation characteristic thatexhibits less variation across the surface of a magnetic recordingmedium. Hence, in accordance with the invention, a more uniformrecording layer thickness can be exploited to achieve improvedsignal-to-noise ratio and increased read/write performance. An improvedsignal-to-noise ratio is particularly advantageous for higher densitymagnetic recording.

The details of one or more embodiments of the invention are set forth inthe description below. Other features, objects, and advantages of theinvention will be apparent from the description and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of a magnetic recording medium.

FIG. 2 is a cross-sectional side view of an alternative magneticrecording medium.

FIG. 3 is a cross-sectional side view of the magnetic recording mediumof FIG. 1 illustrating the thickness of a magnetic recording layer.

FIG. 4 is a hysteresis plot illustrating remanent magnetization andcoercivity in a magnetic recording medium.

FIG. 5 is a side view of a slide coating apparatus useful inmanufacturing a magnetic recording medium.

FIG. 6 is a block diagram of a magnetic recording system.

DETAILED DESCRIPTION

FIG. 1 is a cross-sectional side view of a magnetic recording medium10A. As shown in FIG. 1, magnetic recording medium 10A may include asubstrate 12, a carrier layer 14 formed over the substrate, and amagnetic recording layer 16 formed over the carrier layer. In someembodiments, one or more additional layers may be formed over magneticrecording layer 16. Magnetic recording medium 10A may take the form of amagnetic tape or a magnetic disk. As will be described, magneticrecording medium 10A may be formed by a multi-layer, wet-on-wet coatingprocess, such as slide coating or dual slot coating, to produce enhanceduniformity of thickness within magnetic recording layer 16.

Enhanced uniformity can be effective in reducing magnetic fluxmodulation in magnetic recording layer 16, i.e., increasing magneticflux uniformity, and thereby improving signal-to-noise ratio of datarecorded in the magnetic recording layer. In turn, increasedsignal-to-noise ratio may permit increased data storage densities. Forexample, magnetic recording layer 16 may have a magnetic flux modulationcharacteristic with a one sigma standard deviation of less than 0.06along the recording direction of magnetic recording medium 10, as wellas a coercivity of greater than or equal to approximately 2000 Oe and,more preferably, greater than or equal to approximately 2400 Oe. In someembodiments, the magnetic flux modulation characteristic may have a onesigma standard deviation of less than 0.05, or even 0.04.

FIG. 2 is a cross-sectional side view of an alternative magneticrecording medium 10B. A magnetic recording medium 10B in accordance withthe invention may include one or more additional layers, such as a layer18 formed over magnetic recording layer 16 in magnetic recording medium10B of FIG. 2. Magnetic recording media 10A, 10B may be referred togenerally herein as magnetic recording media 10. The additional layerswithin magnetic recording medium 10B shown in FIG. 2 may include, forexample, antistatic material, abrasive material that aids the cleaningof recording heads during use, lubricating materials that reducefriction between the magnetic recording head and the surface of themagnetic recording medium, or a combination thereof. Additional fluidlayers may be incorporated in magnetic recording medium 10B, as desiredfor media performance, ease of coatability, or productivity. Thus,functional materials can be incorporated in discrete fluid layers.Alternatively, one or more functional materials can be incorporated in asingle fluid layer that, when dried, forms a multi-functional layer inthe resulting magnetic recording medium.

FIG. 3 is a cross-sectional side view of magnetic recording medium 10Aof FIG. 1 illustrating the thickness T_(mag) of magnetic recording layer16. As shown in FIG. 3, magnetic recording layer 16 has a lower surface19 adjacent an upper surface of carrier layer 14, and an upper surface21 opposite the carrier layer. Upper surface 21 of magnetic recordinglayer 16 may be exposed or coated with an additional layer as describedwith reference to FIG. 2. The thickness T_(mag) represents an averagethickness across magnetic recording layer 16 between lower surface 19and upper surface 21. In accordance with the invention, averagethickness T_(mag) may be less than or equal to approximately 0.15microns and, more preferably, less than or equal to approximately 0.10microns. In addition, as a result of enhanced coating uniformity, thethickness of magnetic recording layer 16 may exhibit a reduction inaverage thickness variation in magnetic recording layer 16. A reducedthickness and reduced thickness variation can be effective in reducingthe modulation, i.e., variation, in the magnetic flux modulationassociated with magnetic recording layer 16. In other words, enhancedthickness uniformity in the recording layer can reduce magnetic fluxmodulation along the length and across the width of a magnetic recordingtape.

FIG. 4 is a hysteresis plot illustrating remanent magnetization andcoercivity in a magnetic recording medium. The magnetic remanence (Mr),or remanent magnetization, is the permanent magnetization that remainsin a magnetic material forming a magnetic recording layer after anexternal magnetic field is removed. Hence, the remanent magnetizationsupports the recording of data. Magnetic flux is a function of theremanent magnetization in recording layer 16 and the thickness of therecording layer. Accordingly, modulation of the thickness of recordinglayer 16 along the recording path can result in modulation of magneticflux, and impact the signal-to-noise ratio of data recorded on and readfrom the magnetic recording medium. Excessive magnetic flux modulationcan degrade signal-to-noise ratio, requiring more energy for effectiverecording. FIG. 4 is provided for illustration of a general case ofremanent magnetization, and is not intended to represent the hysteresisplot for any particular recording material contemplated herein.

As shown in FIG. 4, for an unmagnetized recording material, inducedmagnetization increases along a magnetization curve 20 as an externalmagnetic field is applied. The induced magnetization eventuallysaturates, at Ho. If the external field is reduced, the inducedmagnetization also is reduced, but does not follow the originalmagnetization curve. Instead, the magnetic recording material follows anew magnetization curve 22 that reflects a permanent magnetizationcalled the remanent magnetization Mr. If the external field is furtherreduced (curve 24), the remanent magnetization will eventually beremoved, at Hc. The external field for which the remanent magnetizationgoes to zero is termed the coercivity Hc. As the external fieldcontinues to reverse (curve 26), permanent magnetization of the oppositesign is created, until saturation is reached. Curve 28 traces a pathback to Ho, upon a decrease in the reverse magnetic field and the onsetof a forward magnetic field.

Providing a magnetic recording layer with a more uniform thickness inthe magnetic recording medium, in accordance with the invention, canimprove the magnetic flux modulation characteristic. If the degree ofmagnetic flux varies across the surface of a magnetic recording layer,the magnetizations written to the layer may exhibit a differentsignal-to-noise ratio. Accordingly, the applicable noise floor forreadout must assume a worst case, generally hampering increased storagedensities and recording performance. Magnetic flux can vary, inparticular, due to thickness variations in the magnetic recording layer.For example, this type of variation can influence the amount ofmagnetizable material within a given volume of the magnetic recordinglayer. A magnetic recording medium, as described herein, provides amagnetic recording layer with reduced thickness variation. Consequently,a magnetic recording medium constructed in accordance with the inventioncan provide reduced magnetic flux modulation.

FIG. 5 is a side view of a slide coating system 30 useful inmanufacturing a magnetic recording medium 10, e.g., as shown in FIG. 1or FIG. 2. Slide coating represents a suitable technique for achieving amagnetic recording layer with a reduced thickness and reduced thicknessvariation. Other techniques such as dual slot coating also may beeffective. In general, however, slide coating can provide enhance motionquality and coating uniformity to coating substrate 12. As shown in FIG.5, slide coating system 30 includes a backup roller 32 providedproximate a slide coater 34 to support the backside, i.e., non-coatingside, of a coating substrate 12. Coating substrate 12 takes the form ofa continuous web. Backup roller 32 rotates in the direction of travel ofsubstrate 12. Substrate 12 can be transported relative to slide coater34 between supply and takeup rolls (not shown). Slide coater 34 cansimultaneously coat two or more fluid layers in a stacked arrangementonto substrate 12. Following coating, the layers are dried, e.g., bytransportation of substrate 16 through a drying oven (not shown).

Slide coater 34 may include multiple slide blocks 36, 38, 40, 42. In theembodiment of FIG. 5, slide coater 34 includes four slide blocks. Inother embodiments, slide coater 34 may include fewer or more than fourslide blocks, depending on the number of fluid layers to be coated ontosubstrate 12. In some embodiments, for example, the recording medium tobe manufactured may include only a carrier layer and a recording layer,or only a recording layer. Slide blocks 36, 38, 40, 42 define fluidslots 44, 46, 48 and a combined slide surface 50. First slide block 36is disposed adjacent back-up roller 32, while slide blocks 38, 40, 42are disposed upward from the first slide block 36. Slide blocks 36, 38,40 define a continuous slide surface for flow of coating fluids.

A vacuum box 52 can be provided to adjust the level of negative pressureadjacent slide coating system 20. In particular, vacuum box 52 serves tomaintain a differential pressure across the coating bead 53 between aslide surface 50 and substrate 12, thereby stabilizing coating bead 53.Vacuum box 52 may be coupled to a vacuum source (not shown in FIG. 5)and include an outlet (not shown in FIG. 5) for material recovered fromthe coating area.

A first fluid 56 can be distributed to first slot 44 via a first fluidsupply and a first manifold (not shown in FIG. 5). A second fluid 58 canbe distributed to second slot 46 via a second fluid supply and a secondmanifold (not shown in FIG. 5). A third fluid 60 can be distributed tothird fluid slot 48 via a third fluid supply and a third fluid manifold(not shown in FIG. 5). Thus, in an embodiment as shown in FIG. 5, slidecoater 30 is capable of coating a three-layer fluid construction 62 thatincludes a first fluid layer 64 containing first fluid 56, a secondfluid layer 66 containing second fluid 58, and a third fluid layer 68containing third fluid 60. First fluid layer 64 can be coated ontosubstrate 12, with second fluid layer 66 being coated above first fluidlayer 64, and third fluid layer 68 being coated above second fluid layer66. The first, second and third fluid layers 64, 66, 68 may form,respectively, a sub layer, a recording layer, and a functional layercontaining, e.g., an antistatic or head-cleaning agent. Third fluidlayer 68 may be optional in some embodiments.

Fluids 56, 58, 60 may comprise a solvent plus a solute. Typical solventsmay include, for example, methylene chloride, acetone, methyl ethylketone, methyl isobutyl ketone, ethyl acetate, butyl acetate,cyclohexanone, butyl alcohol, N,N-dimethylformamide, toluene, andmixtures thereof. When the solvent dries, the coating solute remainsbehind. In other words, coatings are applied as liquids for ease ofapplication, but the coatings are dry in the finished product.

The type of solute carried by fluids 56, 58, 60 depends on the type ofcoating to be formed. In the manufacture of magnetic storage media, forexample, the solute may include a plurality of magnetic particles. Themagnetic particles may be acicular or needle-like magnetic particleswith an average length along the major axis of less than about 0.3 mm.Typical acicular particles of this type include, for example, particlesof ferro- and ferromagnetic iron oxides such as gamma-ferric oxide(γ-Fe₂O₃), complex oxides of iron and cobalt, various ferrites andmetallic iron particles.

Alternatively, small tabular particles such as barium ferrites and thelike can be employed. The particles can be doped with one or more ionsof a polyvalent metal such as titanium, tin, cobalt, nickel, zinc,manganese, chromium, or the like. First fluid layer 64 may act as acarrier or “sub” layer for second and third fluid layers 66, 68. In thiscase, the wet thickness of first fluid layer 64 on substrate 12 may besubstantially more than the wet thicknesses of second and third fluidlayers 66, 68.

The widths of fluid slots 44, 46, 48 in a direction transverse to thedirection of flow of 30 fluid layers 64, 66, 68 may be substantiallycommensurate with the width of substrate 12. Slide blocks 36, 38, 40 maybe slightly wider than fluid slots 44, 46, 48. In some embodiments, thewidth of substrate 12 may be on the order of 6 to 30 inches (15.24 cm to76.2 cm). In producing magnetic tape media, substrate 12 may be slitlength-wise following coating into several strips, e.g., one-quarterinch (0.64 cm) in width, to produce continuous lengths of recording tapefor loading into data cartridges. In producing magnetic disk media,disks can be cut or punched from substrate 16 as “cookies,” e.g., 3.5inches (90 mm) in diameter, for loading into floppy diskette housings.In either case, each fluid layer 64, 66, 68 preferably extendswidth-wise to the lateral edges of substrate 12.

In some embodiments, in which second fluid layer 66 forms the magneticrecording layer, second fluid 58 contains magnetic material such asmetal magnetic recording particles. In this case, once dried, secondfluid layer 66 forms a magnetic recording layer on substrate 12. Inother embodiments, the magnetic material can be provided in first fluidlayer 64, or in multiple fluid layers 64, 66, 68 of fluid construction62. For example, multiple layers in fluid construction 62 may formmultiple magnetic recording layers. Alternatively, individual magneticlayers can be arranged to work together as a composite multi-layerrecording film.

As discussed above, third fluid 60 may contain a variety of differentsubstances that contribute to the functional properties of the finishedmagnetic recording medium. In other words, once dried, third fluid layer68 may form a functional layer of the magnetic recording medium. Forexample, third fluid 60 may contain antistatic material, abrasivematerial that aids the cleaning of recording heads during use,lubricating materials that reduce friction between the magneticrecording head and the surface of the magnetic recording medium, or acombination thereof.

Additional slide blocks can be added to slide coater 30 for theintroduction of additional fluid layers, as desired for mediaperformance, ease of coatability, or productivity. Thus, such functionalmaterials can be incorporated in discrete fluid layers. Alternatively,one or more functional materials can be incorporated in a single fluidthat, when dried, forms a multi-functional layer in the resultingmagnetic recording medium.

As further shown in FIG. 5, backup roller 32 provides generally uniformsupport and tension to substrate 12 within vacuum box 52 over asubstantial portion of the circumferential surface of the backup roller.In addition, backup roller 32 can be effective in providing precisecontrol of the speed of travel of substrate 12. In this manner, backuproller 32 offers more effective control of the tension of substrate 12as it travels within slide coating system 20, enabling enhanced coatinguniformity relative to other coating techniques. Backup roller 32 alsosupports substrate 12 to maintain the uniformity of distance within thecoating gap. In addition, the application of different fluid layerssimultaneously by slide coater 34 avoids interaction of multiple wetsurfaces from multiple coating devices, and tends to promote enhancedcoating uniformity. In particular, the enhanced coating uniformity canbe effective in promoting more uniform thickness within the recordinglayer, e.g., second fluid layer 66, thereby reducing magnetic fluxmodulation in the finished magnetic recording media. By reducingfluctuation of the coating thickness, magnetic flux modulation in themanufactured recording medium can be reduced.

A coating formulation suitable for use as second fluid layer 66 may havea formulation and characteristics as described below. In particular, themagnetic recording media preferably includes at least one magnetic layerformed from a magnetic coating composition comprising a binder and aplurality of magnetic particles dispersed within the binder. In additionto the at least one magnetic recording layer formed by second fluidlayer 62, a magnetic recording medium in accordance with the inventionmay also include a nonmagnetic layer formed by first fluid layer 64 froma nonmagnetic coating composition comprising a binder and, optionally,nonmagnetic particles dispersed therein, and a nonmagnetic layer formedby third fluid layer 68.

As mentioned above, a magnetic coating composition suitable forformulation of second fluid layer 66 as well as a nonmagnetic coatingcomposition suitable for formulation of first fluid layer 64 includes abinder. Suitable binders that can be used in the magnetic layer coatingcomposition include, for example, vinyl chloride vinyl acetatecopolymers, vinyl chloride vinyl acetate vinyl alcohol copolymers, vinylchloride vinyl acetate maleic acid polymers, vinyl chloride vinylidenechloride copolymers, vinyl chloride acrylonitrile copolymers, acrylicester acrylonitrile copolymers, acrylic ester vinylidene chloridecopolymers, methacrylic ester vinylidene chloride copolymers,methacrylic esterstyrene copolymers, thermoplastic polyurethane resins,phenoxy resins, polyvinyl fluoride, vinylidene chloride acrylonitrilecopolymers, butadiene acrylonitrile copolymers, acrylonitrile butadieneacrylic acid copolymers, acrylonitrile butadiene methacrylic acidcopolymers, polyvinyl butyral, polyvinyl acetal, cellulose derivatives,styrene butadiene copolymers, polyester resins, phenolic resins, epoxyresins, thermosetting polyurethane resins, urea resins, melamine resins,alkyl resins, urea formaldehyde resins and the like.

The binders may be provided in a suitable non-aqueous solvent, such asmethylene chloride, acetone, methyl ethyl ketone, methyl isobutylketone, ethyl acetate, butyl acetate, cyclohexanone, butyl alcohol,N,N-dimethylformamide, toluene, and mixtures thereof. Preferred bindersinclude polyurethanes, non-halogenated vinyl copolymers, halogenatedvinyl copolymers, and a combination thereof. As used herein, the term“nonhalogenated” means that the copolymer contains no covalently boundhalogen atoms. Thus, the term “nonhalogenated” excludes vinyl halidemonomers such as vinyl chloride or vinylidene chloride as monomericcomponents of the copolymer, but the term “nonhalogenated” does notexclude monomeric components such as (meth)acryloyloxyethyltrimethylammonium chloride in which chlorine is present as a chlorideanion. As used herein, the prefix “(meth)acryl-” means “methacryl-” or“acryl-”. The term “vinyl” with respect to a polymeric material meansthat the material comprises repeating units derived from vinyl monomers.As used with respect to a vinyl monomer, the term “vinyl” means that themonomer contains a moiety having a free-radically polymerizablecarbon-carbon double bond. Monomers having such moieties are capable ofcopolymerization with each other via the carbon-carbon double bonds.

In the invention, one useful polyurethane is a carboxyl polyurethanepolymer, such as that described in U.S. Pat. No. 5,759,666 (Carlson etal.). The carboxyl polyurethane polymer typically comprises the reactionproduct of a mixture comprising: (i) one or more polyisocyanates, (ii) acarboxylic acid functional polyol, and, (iii) optionally, one or morepolyols defined to exclude the former carboxylic acid functional polyol,wherein the number of isocyanate-reactive groups present in the mixtureprior to reaction exceeds the number of isocyanate groups and at leastabout 0.2 meq of carboxylic acid groups are present on the carboxylpolyurethane polymer per gram of carboxyl polyurethane polymer.Typically, the reaction product has a number average molecular weightfrom about 2000 to about 50,000, preferably from about 5000 to about30,000.

The term “polyol,” as used herein, refers to polyhydric alcoholscontaining an average of one or more hydroxyl groups and includes,monohydric alcohols, diols, triols, tetrols, etc. Preferred polyols arediols, that include both low molecular weight (i.e., having less thanabout 500 number average molecular weight) and oligomeric diols,typically having a number average molecular weight from about 500 toabout 5000. Representative examples of low molecular weight diolsinclude, but are not limited to, ethylene glycol, propylene glycol,diethylene glycol, diols having polar functional groups, diols bearingethylenic unsaturation (e.g., 3-allyloxy-1,2-propandiol, 1-glyceryl(meth)acrylate, etc.) and fluorinated diols. Representative examples ofoligomeric diols include, but are not limited to, polyether diols,polyester diols, polyether triols, and polyester triols.

The term “polyisocyanate,” refers to any organic compound that has twoor more reactive isocyanate (i.e., —NCO) groups in a single moleculethat can be aliphatic, alicyclic, aromatic, and a combination thereof,and includes diisocyanates, triisocyanates, tetraisocyanates, etc., anda combination thereof. Preferably, diisocyanates are used and includediphenylmethane diisocyanate, isophorone diisocyanate, toluenediisocyanate, hexamethylene diisocyanate, tetramethylxylenediisocyanate, p-phenylene diisocyanate, and a combination thereof.

Another useful polyurethane is a phosphonated polyurethane, such asdescribed in U.S. Pat. No. 5,501,903 (Erkkila et al.). Preferably, thephosphonated polyurethane includes nitrogen forming part of the backboneof the polymer, a single bond or divalent linking group (preferablyincluding up to 4 linear carbon atoms), and two pendant groupsindependently selected from the group of an alkyl group, a cycloalkylgroup, an aryl group, or together comprise the necessary carbon atoms tocomplete a ring. The phosphonated polyurethane is preferably formed byreaction of a soft segment diol in which the hydroxyl groups areseparated by a flexible chain (typically having a molecular weight ofmore than 300, and includes a polycaprolactone diol, for example), ahard segment diol in which the hydroxyl groups are separated by arelatively inflexible chain (typically having a molecular weight of lessthan 300, and includes neopentyl glycol, for example), a triol (e.g., apolycaprolactone triol), a diisocyanate (e.g., toluene diisocyanate,4,4-diphenylmethane diisocyanate, and isophorene diisocyanate), and adialkyl phosphonate (e.g., diethylbis-(2-hydroxyethyl)aminomethylphosphonate).

An example of a useful quaternary ammonium-containing polyurethane is apolymeric quaternary ammonium compound described in U.S. Pat. No.5,759,666 (Carlson et al.). In particular, polymeric quaternary ammoniumcompounds preferably have a number average molecular weight greater thanabout 500, preferably selected from the group of a quaternary ammoniumpolyurethane, a quaternary ammonium functional non-halogenated vinylcopolymer, and a combination thereof.

A suitable binder may include a quaternary ammonium functionality. Asused herein, the term “quaternary ammonium functionality” refers tomoieties of the formula

In the formula above, the bond denoted with the asterisk is attached tothe backbone of the polymeric binder resin either directly or indirectlythrough a difunctional linking group; each R may independently be anysuitable moiety or co-member of a ring structure, and is preferably H oran alkyl group of 1 to 10 carbon atoms such as —CH₃; and M is anysuitable counter anion such as Cl⁻, BR⁻, or the like. The term“quaternary ammonium functionality” also would encompass sulfobetaines,(e.g., —N⁺(CH₃₎ ₂(CH₂CH₂CH₂SO₃ ⁻)).

In one embodiment, the quaternary ammonium functional polymer is anonhalogenated vinyl copolymer which is incorporated into the polymericbinder as the so-called “hard resin” component having a relatively highglass transition temperature (T_(g)).

In another embodiment, the nonhalogenated, vinyl copolymer is of thetype comprising a plurality of pendant quaternary ammonium groups, aplurality of pendant crosslinkable moieties such as OH groups ormoieties having carbon-carbon double bonds, and a plurality of pendantnitrile groups. Without wishing to be bound by theory, it is believedthat the nitrile groups may promote the compatibility of these vinylcopolymers with polyurethanes. It is also believed that the pendanthydroxyl groups of the vinyl copolymer not only facilitate dispersion ofthe magnetic pigment in the polymeric binder, but also promotesolubility, cure, and compatibility with other polymers. The quaternaryammonium groups of the vinyl copolymer facilitate dispersion of themagnetic pigment in the polymeric binder.

In yet another embodiment, the quaternary ammonium functional polymer isa quaternary ammonium polyurethane that has at least one quaternaryammonium group pendant from a polyurethane chain of molecular weightgreater than about 500.

Another useful non-halogenated vinyl copolymer is one having a pluralityof pendant nitrile groups, a plurality of pendant hydroxyl groups, andat least one pendant dispersing group, as described in U.S. Pat. No.5,501,903 (Erkkila et al.) and U.S. Pat. No. 5,510,187 (Kumar et al.),for example. The non-halogenated vinyl copolymer having a plurality ofpendant nitrile groups, a plurality of pendant hydroxyl groups, and atleast one pendant dispersing group is preferably a nonhalogenated vinylcopolymer of monomers comprising 5 to 40, preferably 15 to 40, parts byweight of (meth)acrylonitrile; 30 to 80 parts by weight of one or morenonhalogenated, nondispersing, vinyl monomers; 1 to 30 parts by weightof a nonhalogenated, hydroxyl functional, vinyl monomer; and 0.25 to 10parts by weight of a nonhalogenated, vinyl monomer bearing a dispersinggroup. The dispersing group can be selected from quaternary ammonium,acid or salt of carboxyl, acid or salt of phosphate or phosphonate, acidor salt of sulfate or sulfonate, and mixtures thereof. When thedispersing group is quaternary ammonium, it is preferred that the vinylmonomer bearing a dispersing group is (meth)acryloyloxyethyltrimethylammonium chloride.

Preferably, the nonhalogenated, nondispersing, vinyl monomer is selectedfrom styrene; an alkyl ester of (meth)acrylic acid wherein the alkylgroup of the alkyl ester has 1 to 20 carbon atoms; and a blendcomprising styrene and such an alkyl ester (e.g., methyl (meth)acrylate,more preferably methyl methacrylate) wherein the weight ratio of styreneto the alkyl ester is in the range from 10:90 to 90:10.

Significantly, halogenated vinyl copolymers are also useful, andsusceptible to slide coating in accordance with the invention. Theseinclude vinyl chloride resins, vinyl chloride-vinyl acetate resins,vinyl chloride-vinyl acetate-vinyl alcohol resins, vinyl chloride-vinylacetate-maleic anhydride resins, and a combination thereof, such asthose described in U.S. Pat. No. 5,763,046 (Ejiri et al.). Preferably,these resins also include one or more polar groups bonded thereto.Preferred polar groups include SO_(3 M) ¹, COO M¹, OSO₃M¹, P═O(O M²)OM³, —OP═O(O M²)O M³, —NRX, OH, NR₁, N⁺R₂ (wherein R is a hydrocarbongroup), an epoxy group, SH, and CN. One more useful type of vinylchloride resin is a vinyl chloride copolymer containing epoxy groups,e.g., a copolymer containing a vinyl chloride repeating unit, anepoxy-containing repeating unit, and, if desired, a polargroup-containing unit (e.g., —SO₃M, —OSO₃M, —COOM, and —PO(OM)², whereinM is hydrogen or an alkali metal). Of these, a copolymer containing arepeating epoxy group and a repeating unit containing —SO₃Na areparticularly useful.

The polymers mentioned above may be prepared by polymerization methodsknown in the art, including but not limited to bulk, solution, emulsionand suspension free-radical polymerization methods. For example,according to the solution polymerization method, copolymers are preparedby dissolving the desired monomers in an appropriate solvent, adding achain-transfer agent, a free-radical polymerization initiator, and otheradditives known in the art, sealing the solution in an inert atmospheresuch as nitrogen or argon, and then agitating the mixture at atemperature sufficient to activate the initiator.

A variety of additives known to those skilled in the art can beincorporated into the dispersions and coatings described herein. Thedispersions and coatings can further comprise additives including butnot limited to those selected from the group consisting of crosslinkers,head-cleaning agents, lubricants, carbon black, dispersants, and wettingagents.

For example, if desired, a binder composition may also include acrosslinker. One preferred type of crosslinker is a polyisocyanatecrosslinker known to the magnetic recording media art cure to a glasstransition temperature of greater than about 100° C. and may be used toproduce layers of high glass transition temperature and hardness. Aparticularly useful type of polyisocyanate crosslinker is the reactionproduct of an excess of a diisocyanate with low number average molecularweight (under about 200) diols and triols. A typical and widely usedcurative comprises, for example the adduct of toluene diisocyanate witha mixture of trimethylol propane and a diol such as butane diol ordiethylene glycol. A preferred material of this type is available underthe trade designation MONDUR CB-55N from Bayer Corporation. Other usefulhigh Tg crosslinkers are available under the trade designations MONDURCB-601, MONDUR CB-701, MONDUR MRS, and DESMODUR L (all available fromBayer Corporation) and CORONATE L (available from Nippon Polyurethane).Additional isocyanate crosslinking agents are described in U.S. Pat. No.4,731,292 (Sasaki et al.).

A toughened polyisocyanate crosslinker which cures to a tough andflexible, rather than a brittle, film may be desirable. Useful toughenedpolyisocyanate crosslinkers are described in U.S. Pat. No. 5,759,666(Carlson et al.) and are obtained as the reaction product of an excessof a polyisocyanate with polyols, including 10-80% by weight of anoligomeric polyol which acts as a toughening segment. The oligomericpolyols useful in making toughened polyisocyanate curatives have anumber average molecular weight of about 500 to about 5000 and a glasstransition temperature of lower than about 0° C., preferably lower thanabout minus 20° C. The oligomeric polyols are preferably selected fromthe group consisting of a polyester diols, polyester triols, polyetherdiols, polyether triols, polycarbonate diols, polycarbonate triols, andmixtures thereof.

One of the preferred toughened polyisocyanate crosslinkers is made fromthe reaction product of CB-55N (described above), with 45 weight percentof a polycaprolactone diol of 1300 number average molecular weight. Thismodification of CB-55N provides a faster cure and a tougher coating. Itis preferred in formulations in the dispersions and coatings of theinvention to use between about 20 and about 60 weight percent, mostpreferably about 30 to about 50 weight percent of the toughenedpolyisocyanate curative based upon the weight of formulation solidsexclusive of particles.

As mentioned above, other additives include head-cleaning agents,lubricants, carbon black, dispersants, and wetting agents. It isenvisioned that lubricants such as those disclosed in U.S. Pat. No.4,731,292 (Sasaki et al.), U.S. Pat. No. 4,784,907 (Matsufuji et al.),and U.S. Pat. No. 5,763,076 (Ejiri et al.) could be added to obtaindesired frictional and processing characteristics. Examples of usefullubricants include but are not limited to those selected from the groupconsisting of C₁₀ to C₂₂ fatty acids, C₁ to C₁₈ alkyl esters of fattyacids, and mixtures thereof. Other examples of useful lubricants includethose selected from the group consisting of silicone compounds such assilicone oils, fluorochemical lubricants, fluorosilicones, andparticulate lubricants such as powders of inorganic or plasticmaterials. Preferred lubricants include those selected from the groupconsisting of myristic acid, stearic acid, palmitic acid, isocetylstearate, oleic acid, and butyl and amyl esters thereof. Typicallymixtures of lubricants are used, especially mixtures of fatty acids andfatty esters.

The dispersion may further comprise about 1 to about 10 weight percentof a wetting agent based upon the weight of the magnetic particles.Suitable wetting agents include but are not limited to those selectedfrom the group consisting of phosphoric acid esters such asmono-phosphorylated propylene oxide adducts of glycerol, e.g., thereaction product of 1 mole of phosphorous oxychloride with the reactionproduct of 10-11 moles of propylene oxide and 1 mole of glycerine.

Examples of useful head cleaning agents include but are not limited tothose disclosed in U.S. Pat. No. 4,784,914 (Matsufuji et al.) and U.S.Pat. No. 4,731,292 (Sasaki et al.). Examples of such head cleaningagents include but are not limited to those selected from the groupconsisting of alumina, chromium dioxide, alpha iron oxide, and titaniumdioxide particles of a size less than about 2 microns, preferably lessthan 0.5 microns, which have a Mohs hardness of greater than about 5 andwhich are added in an amount ranging from about 0.2 to about 20 partsper hundred parts of magnetic particles.

As mentioned above, a magnetic layer contains a plurality of magneticparticles. Preferably, the magnetic particles are acicular or needlelike magnetic particles. The average length of these particles along themajor axis preferably is less than about 0.3 μm, and more preferably,less than about 0.2 μm. The particles preferably exhibit an axial ratio,that is, a length to diameter thickness ratio of up to about 5 or 6to 1. Preferred particles have a specific surface area of at least about30 m²/g, more preferably of at least about 40 m²/g. Typical acicularparticles of this type include, for example, particles of ferro- andferromagnetic iron oxides such as gamma-ferric oxide (γ-Fe₂O₃), complexoxides of iron and cobalt, various ferrites and metallic iron particles.Alternatively, small tabular particles such as barium ferrites and thelike can be employed. The particles can be doped with one or more ionsof a polyvalent metal such as titanium, tin, cobalt, nickel, zinc,manganese, chromium, or the like as is known in the art. The magneticparticles can be present in the dispersion in an amount of from about50% to about 90% by weight, preferably about 60% to about 80% by weight.

A preferred particle is a magnetic alloy particle having highcoercivities and high saturation magnetization that preferably includeabout 15 to 45 atomic %, preferably 20 to 45 atomic percent, Co based onthe amount of Fe present (i.e., 100×(atoms of Co/atoms of Fe)).Preferably, these alloy particles have coercivities greater than about1800 Oersteds (Oe), more preferably, from about 1800 to about 2500 Oe,and even more preferably, about 2000 to about 2400 Oe. The saturation ofmagnetization of the alloy particles is preferably greater than or equalto 130 emu/g and, more preferably, greater than 135 emu/g. Such metalalloy particles can be prepared by the method described in U.S. Pat. No.5,735,969 (Lown et al.), and are commercially available from DowaMining, Kanto Denka, and Toda Kogyo Corporation, for example.

Magnetic particles for use in the invention may incorporate at least afirst surface treatment agent that is desirably adsorbed onto thesurfaces of the magnetic pigment. The surface treatment agent is acompound comprising at least one acidic group and at least one electronwithdrawing group. Advantageously, the use of a surface treatment agentwith this kind of multiple functionality improves the dispersability ofmagnetic pigments in polymeric binders having quaternary ammoniumfunctionality. As a result, the corresponding magnetic recording mediaare easier to manufacture and have better electromagnetic and mechanicalperformance properties than if the surface treatment agent lacked one orboth of the acid or electron withdrawing functionalities.

A wide variety of acidic groups may be used as the acidic group of thesurface treatment agent of this invention with beneficial results.Representative examples of suitable acidic groups include an anhydricgroup, a —COOH group, sulfonic acid, a phosphonic acid group, salts ofsuch groups, combinations of these, and the like. Of these, —COOH ispresently most preferred in combination with metal particle magneticpigments. In the practice of the invention, a salt of an acidic group isalso deemed to be an acidic group within the scope of the invention.

As used herein, the term “electron-withdrawing” group is a group which,if substituted for a Hydrogen atom (other than the acidic H) on acarboxylic acid would make the acid have a lower pKa, i.e. thefunctional group has a Hammett Substituent Constant greater than 0.1 asdescribed in Introduction to Organic Chemistry, Andrew Streitwieser, Jr.and Clayton H. Heathcock, McMillan Publishing Co., Inc. (NY, N.Y. 1976)pp. 947-949. Representative examples of electron withdrawing groupsinclude nitro, chloro, bromo, fluoro, iodo, oxo, perfluoroalkyl (such astrifluoromethyl), perfluoroalkoxy, hydroxy, cyano, combinations ofthese, and the like.

The magnetic layer desirably incorporates a sufficient amount of thesurface treatment agent effective to ease dispersion and help preventagglomeration of the magnetic pigment during preparation of the magneticrecording medium of this invention. The optimum amount of surfacetreatment agent will depend upon a number of factors including the acidequivalent weight of the surface treatment agent, the specific surfacearea of the magnetic pigment being surface treated, the pH of magneticpigment being treated, and the like.

In one preferred embodiment, the surface treatment agent is a compoundhaving the formulaE-X-Awherein E is the electron withdrawing group, A is the acidic group, andX comprises an aromatic moiety. Preferably, X is an aromatic ring, and Eand A are substituents of the aromatic ring at meta or para positionsrelative to each other. More preferably, E and A are at a para positionrelative to each other. Due to greater spacing between the E and Agroups, the surface treatment agent is much more effective when E and Aare at a meta or para position relative to each other as compared to theperformance of the agent if E and A were to be ortho to each other.

Preferably, the plurality of magnetic particles are first prepared as aconcentrated magnetic particle dispersion prior to its addition to thebinder. The concentrated magnetic particle dispersion can be prepared byprocedures known to those in the dispersion art. The dispersion can beprepared by the use of a dispersing machine, for example, a high speedimpeller mill, an attritor, or a sand mill.

The concentrated magnetic particle dispersion can be diluted with asuitable non-aqueous organic solvent to make a magnetic coatingcomposition. Typically, the non-aqueous organic solvent has dissolved ordispersed therein a binder, as described above. Solvents useful fordilution of the concentrated magnetic dispersion include ketones such asacetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone,isophorone; esters such as methyl acetate, ethyl acetate, butyl acetate,isobutyl acetate, isopropyl acetate, ethyl lactate, glycol monoethylether acetates; ethers such as diethyl ether, tetrahydrofuran, glycoldimethyl ethers, dioxane; aromatic hydrocarbons such as benzene,toluene, xylene, cresol, chlorobenzene, styrene; chlorinatedhydrocarbons such as methylene chloride, ethylene chloride, carbontetrachloride, chloroform, ethylene chlorohydrin, dichlorobenzene;N,N-dimethylformamide, and hexane.

In some embodiments of the invention, it may be desirable to incorporatethe binder described herein in a coating composition that is notrequired to possess magnetic properties, such as a primer/adhesionpromoting layer, an activator layer, a sub layer (typically locatedbetween the magnetic layer and the substrate), and a top layer. Forexample, a coating composition for forming a sub layer such as firstfluid layer 60 can comprise non-magnetizable particles, such as, forexample, those selected from the group consisting of carbon black,alpha-iron oxide, aluminum oxide, titanium dioxide, zinc oxide, silicagel, calcium carbonate, barium sulfate, and mixtures thereof.

A variety of additives, such as those listed above, may also beincorporated into a non-magnetic layer such as third fluid layer 64. Asuitable additive can include head-cleaning agents, lubricants, carbonblack, dispersants, wetting agents, and the like.

EXAMPLE

All materials used in this example are readily available from standardcommercial sources, unless otherwise specified. All percentages are byweight unless otherwise indicated. Coating formulations were preparedfor both a first fluid layer and a second fluid layer to be coated on asubstrate to form a magnetic recording medium. The first fluid layerserved as a sub layer for application to the substrate. The second fluidlayer was deposited on top of the sub layer and formed a recordinglayer. The first fluid layer (the sub layer) was formed with thefollowing formulation:

PPH - Metal Pigment Solution Mill Additions DPN 250 alpha iron oxide100.00 100.00 (metal pigment - 0.15 micron length) BP2000 carbon black6.00 6.00 K32 (Styrene acronitrile copolymer) 17.19 39.06 Z2 Urethaneresin 8.59 21.49 Myristic Acid 0.50 0.50 Butyl Stearate 0.50 0.50 MethylEthyl Ketone 23.25 Methyl Isobutyl Ketone 136.64 Tetrahydrofuran 106.59Mill Solids (30.6%) 132.78 434.03 Coating Additions Butyl Stearate 0.500.50 PPA45 (isocyanate-containing 6.44 12.89 curative - 50% Solids)Methyl Ethyl Ketone 18.00 Methyl Isobutyl Ketone 40.49 Tetrahydrofuran31.49 Coating Solids (26%) 139.72 537.40The alpha iron oxide was obtained in a 0.15 micron length from TodaKogyo Corp. The BP 2000 (Black Pearls 2000) was obtained from CabotCorporation. The myristic acid and butyl stearate were both obtainedfrom Henkel Canada Ltd.

The second fluid layer served as a recording layer, and was formed withthe following formulation:

PPH-Metal Pigment Solution Mill Additions HM-101 magnetic pigment (Hc1830, 0.1 micron) 100.00 100.00 Ceralox APA 0.4 Alumina 19.73 30.50 BP2000 carbon black 0.50 0.50 4-Nitrobenzoic Acid 4.00 4.00 K32 (44%solids) styrene acronitrile copolymer 6.33 13.64 Z2 (40% solids)urethane resin 6.00 15.00 Methyl Ethyl Ketone 126.50 Methyl IsobutylKetone 60.06 Tetrahydrofuran 31.30 Mill Solids (35.8%) 136.57 381.50Coating Additions Myristic Acid 0.50 0.50 Butyl Stearate 1.00 1.00 PPA45(isocyanate-containing 6.90 13.73 curative - 50% Solids) Methyl EthylKetone 213.16 Methyl Isobutyl Ketone 81.98 Tetrahydrofuran 32.79 CoatingSolids (20.0%) 144.97 724.67The magnetic pigment was obtained from Dowa Mining Co., Ltd. The4-nitrobenzoic acide was obtained from Nordic Synthesis AB. The aluminawas obtained from the Ceralox Division of Condea Vista.

A number of samples of the above liquid coating formulations were coatedonto a substrate to form a magnetic recording medium. Following drying,the magnetic recording layer exhibited the characteristics indicated inTable 1 below.

TABLE 1 SAMPLE 1 2 3 4 5 Coater Slide Slide Slide Slide Slide Hc 24442425 2430 2532 2573 SFD .044 0.39 0.40 0.40 0.42 Phi-R 0.4022 0.46400.5085 0.4314 0.1986 Phi-M 0.5000 0.5611 0.6072 0.4952 0.2372 SQ Ratio0.804 0.827 0.837 0.871 0.837 Tmag 0.089 0.099 0.11 .086 0.043 Mr Delta2.17 2.22 2.58 2.28 1.19 Sigma 0.062 0.07 0.055 0.046 0.034

In Table 1, the coater refers to the type of coater used in forming themagnetic recording medium, i.e., slide versus die coater. In thisExample, a slide coater similar to that depicted in FIG. 5 was used. InTable 1, H_(c) refers to the measured coercivity of the recording layer,SFD refers to switching field distribution, SQ Ratio refers to thesquareness of the hysteresis curve and thus the ratio of remanentmagnetization to coercivity, Tmag refers to the average thickness of themagnetic recording layer in a recording direction in microns, MR Deltarefers to the variation in the remanent magnetization along therecording direction, and Sigma refers to the standard deviation of theremanent magnetization.

As shown in Table 1, the resulting magnetic recording layer exhibited amagnetic flux modulation (Mr Delta) characteristic with a deviation ofless than or equal to 0.07 for samples 1 and 2 and less than 0.06 forsamples 3, 4 and 5. Indeed, sample 4 produced a standard deviation ofless than 0.05, and sample 5 produced a standard deviation of less than0.04.

FIG. 6 is a block diagram of a magnetic recording system 76 that mayinclude a magnetic recording medium 10 as described herein. Inparticular, magnetic recording system 76 may incorporate a magnetic tapeexhibiting a reduced magnetic flux modulation characteristic. As shownin FIG. 6, magnetic recording system 76 may include a tape drive housing78 that receives a tape cartridge. Tape cartridge 80 contains a tapereel 82 that delivers magnetic tape to a takeup reel 86 within tapedrive housing 78. Tape 84 may travel along a tape path defined by aplurality of tape guides 88A-88D.

The tape path brings tape 84 into proximity with a magnetic read/writehead assembly 90. Head assembly 90 includes a write head that formsmagnetizations along tracks on tape 84 to store data. In addition, headassembly 90 includes a read head that reads the magnetizations from tape84 to retrieve data. A controller 92 controls the operation of a currentdrive circuit that drives head assembly 90. Controller 92 communicateswith a host computer 94 via a computer interface 96, e.g., PCI, USB,SCSI, IEEE 1394, or the like. In operation, magnetic recording system 76may benefit from enhanced performance as a result of the reducedremanent magnetization and increased signal-to-noise ratio provided bymagnetic tape 84, as described herein.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Otherembodiments are within the scope of the following claims.

1. A magnetic tape recording system comprising: a magnetic tapeincluding a substrate and a magnetic recording layer formed over thesubstrate, wherein the magnetic recording layer has an Mr Deltaexpressed in milli-electromagnetic units per centimeter squared(memu/cm²) between approximately 1.19 and 2.58 memu/cm² with a standarddeviation of the Mr Delta between approximately 0.034 and 0.07 memu/cm²;a magnetic head; and a controller that controls the magnetic bead toread data from and write data to the magnetic tape medium.
 2. Themagnetic tape recording system of claim 1, wherein the magneticrecording layer has an average thickness of less than or equal toapproximately 0.15 microns.
 3. The magnetic tape recording system ofclaim 1, Wherein the magnetic recording layer has an average thicknessof less than or equal to approximately 0.10 microns.
 4. The magnetictape recording system of claim 1, wherein the magnetic recording layerexhibits a coercivity of greater than approximately 2000 Oe.
 5. Themagnetic tape recording system of claim 1, wherein the magneticrecording layer has a coercivity of greater than approximately 2300 Oe.6. The magnetic tape recording system of claim 1, wherein the magneticrecording layer includes metal magnetic particles dispersed in apolymeric binder.
 7. The magnetic tape recording system of claim 1,wherein the standard deviation of the Mr Delta is less than or equal toapproximately 0.05 memu/cm².
 8. The magnetic tape recording system ofclaim 1, wherein the standard deviation of the Mr Delta is less than orequal to approximately 0.04 memu/cm².
 9. The magnetic tape recordingsystem of claim 1, the magnetic tape medium further comprising a carrierlayer formed between the substrate and the magnetic recording layer. 10.The magnetic tape recording system of claim 1, wherein the Mr Delta isbetween approximately 1.19 and 2.28.
 11. The magnetic tape recordingsystem of claim 10, wherein the Mr Delta is between approximately 1.19and 2.17.
 12. The magnetic tape recording system of claim 1, wherein thehead has sufficient sensitivity to acquire a useful signal-to-noiseratio (SNR) from the magnetic tape medium that includes the magneticrecording layer that exhibits the Mr Delta between approximately 1.19and 2.58.
 13. The magnetic tape recording system of claim 1, wherein themagnetic recording layer exhibits a coercivity of geater thanapproximately 2000 Oe and the head generates sufficient fields to recordon the magnetic recording layer.
 14. A magnetic tape recording systemcomprising: a magnetic tape including a substrate and a magneticrecording layer formed over the substrate, wherein the magneticrecording layer has an Mr Delta expressed in milli-electromagnetic unitsper centimeter squared (memu/cm²) between approximately 1.19 and 2.58memu/cm², wherein a standard deviation of the Mr Delta is less thanapproximately 0.07 memu/cm². a magnetic head; and a controller thatcontrols the magnetic head to read data from and write data to themagnetic tape.
 15. The magnetic tape recording system of claim 14,wherein the Mr Delta is between approximately 1.19 and 2.17 memu/cm².16. The magnetic tape recording system of claim 14, wherein the magneticrecording layer has a coereivity of greater than approximately 2300 Oe.17. The magnetic tape recording system of claim 14, further comprising acarrier layer formed between the substrate and the magnetic recordinglayer.